During my worst times with anxiety, the mornings have been, bar none, the most difficult part of my day. I would wake up with an immediate knot in my stomach, heart racing (need to go to the bathroom), sort of shaky, etc. In fact, in the past, it was always a harbinger of a bad anxiety episode/depression episode for me that I would start waking up for a string of mornings with that knot in my stomach. It would sometimes happen before the episode became full blown - almost like a warning signal. And, yet, by the end of the day, I would usually feel much better and my body not as keyed up (depending on how severe the anxiety episodes was at that time).
I remember asking my psychiatrist at the time what it is about morning anxiety that makes it seem like a very predictable aspect of an anxiety disorder. He said that if we are able to sleep with some semblance of peace, when we wake up it's like a shock to an overly nervous system to come back into consciousness after being somewhat relaxed (if that was even possible) during sleep. Kind of like a glass of cold water in the face.